This invention relates generally to processing data acquired from three-dimensional imaging systems, and more particularly to enhancing the dynamic range associated with data acquired from imaging systems using low-power sequential analog-to-digital conversion, distributed in space and time, including such systems implemented in CMOS.
Imaging sensors are used in a variety of applications, including cameras, video, radar systems, and instrumentation. In generally, such sensors rely upon detection of electromagnetic (EM) energy, for example emitted energy that is reflected off of a target object and then detected.
Various techniques are used to detect EM energy, varying upon the application, the energy wavelength, and the desired speed of data acquisition. For example, in radar systems, EM waves are generally focused by a maze of waveguides and detected within a cavity using a diode that is sensitive to the frequency of interest.
In light and infrared (IR) imaging systems, EM waves that are reflected from a target object are collected by photodiode detectors in the form of a small photodiode current. This current can then be used to charge a relatively large capacitor over a long integration time period. After integration time ends, the signal voltage developed across the capacitor will eventually be read out, for example via sequential charge transfer as in charge coupled devices (CCDs). The signal voltage representing target objects farther away will be lower in general that the signal voltage representing nearby target objects.
An especially useful IR-type sensing application with which the present invention may (but need not) be practiced will now be described for background purposes. One form of IR-type sensing is so-called three-dimensional sensing and is described in U.S. Pat. No. 6,323,942 entitled xe2x80x9cCMOS-Compatible Three-Dimensional Image Sensor ICxe2x80x9d (2001), assigned to assignee herein. The ""942 patent discloses the use of light emissions and measurements of partial energy reflected from a target a distance Z away to determine distance between such a sensor system and the target.
FIG. 1 depicts a generic IR image sensing system 10 such as that described in the ""942 patent, a system that can determine distance Z between system 10 and a target object 20. Much of system 10 is implemented on a CMOS IC 30 that includes an array 40 of pixel detectors 50 (e.g., photodiodes), and dedicated electronics 60 preferably associated with each pixel detector. An optical energy emitter 70, e.g., a LED or laser diode, emits energy via a lens 80, some of which energy is reflected from the target object 20 and can be detected by at least some pixel detectors in array 40. (Emitter 70 may in fact be implemented off-IC 30.) Every pixel detector within the array captures the partial energy of the light being reflected by a point on the target""s surface and thus captures the distance from the pixel detector to such point.
IC 30 includes a microprocessor or microcontroller unit 90, memory 100 (which preferably includes random access memory or RAM and read-only memory or ROM memory, and various input/output (I/O) and interface circuits, collectively 110. Microprocessor 90 controls operation of the energy emitter 70 and of the various electronic circuits within IC 30. Using various signal processing techniques, the time-of-flight (TOF) for optical energy to travel from system 10 to a point on target object 20 and be at least partially reflected back via an optional lens 120 to a pixel detector 50 within array 40 can be determined. This determination is often termed TOF acquisition. Since the speed of light is known, the distance Z associated with a given time measurement can be determined, e.g., perhaps time t1 is associated with a distance Z1, whereas a longer time t3 is associated with a more remote distance Z3, etc. One can construct a three-dimensional image of a target or scene by combining the data collected from every pixel in the array. Various raw data (DATA) can of course be exported off-IC for further and perhaps more extensive signal processing.
Within detector array 40, the measurement of incoming light energy reaching a given pixel detector is known as brightness acquisition. Various techniques for TOF acquisition and/or brightness acquisition useable in multi-dimensional image sensing exist. A very practical problem encountered with sensing systems, including those described above, is that the peak power of the light energy that is detected may vary by several orders of magnitude, e.g., representing information from a very dim surface point to representing information from a very bright surface point of the target object.
In imaging devices such as above-described, the ratio between the highest and lowest measurable EM energy is limited by the lowest detectable energy in the EM wave, and by the saturation voltage across the integration capacitor. The simultaneous detection of very dim and strong sources of light using the same mechanism is generally performed using two techniques, namely automatic gain control (AGC) and over-sampling.
On one hand, AGC techniques employ an automatic gain control preamplifier that adjusts amplifier gain level so as to keep the amplified photodiode signal within a predefined range. The readout data includes both the amplifier output and the gain value, and can be interpreted as the mantissa and exponent of the desired output signal. In various CCD device applications, AGC techniques have been developed to cope with dynamic ranges of about 35 dB.
On the other hand, over-sampling techniques include comparing the amplified signal with a pre-defined threshold, and resetting the signal and generating a pulse when the threshold is attained. Such generated pulses form a continuous stream of bits that can be coded onto digital words representing the amplified photodiode signal. This second technique is analogous to a class of over-sampling analog-to-digital converters (ADCs) known as sigma-delta (or delta-sigma) converters.
Acquisition of information detected by the pixel array 40 in FIG. 1 may be performed in two phases: a first phase directed to brightness acquisition, and a second phase directed to TOF acquisition.
In a first (brightness acquisition) phase, incoming pulses of light energy are captured by photodiodes or pixel detectors 50 within array 40, which detectors translate the photon energy into detector current. The detector current from each pixel can be integrated over a variable amount of time to create an output signal voltage pulse. Eventually the integrated voltage signal level reaches a given threshold, at which time the integration period ends and a logic pulse is generated for use in incrementing a logic counter. At the end of acquisition, the logic counter holds a logic state uniquely representing the total number of received logic pulses. The brightness of light at a given pixel is proportional to such state. This first phase is performed simultaneously and independently in a matrix array of Nxc3x97M points of acquisition or pixels.
In a second (TOF acquisition) phase, the time delay between the energy pulse emitted by emitter 70 and the target-reflected received pulse detector within array 40 is automatically matched to a normalized value. The signal voltage associated with such value will be a measure of the TOF, which measure can be stored in the very same logic counter noted described above.
The logic counter-held digital content for each pixel in the array may be accessed sequentially or randomly, and the overall image detected by the array can subsequently be decoded and stored in local random access memories (RAMs), e.g., associated with memory 100 in FIG. 1. The RAM contents can then be uploaded to a personal computer or other device using standard communication links, e.g., wireless links, wired links, etc.
As noted above, reflected incoming energy may represent a very bright region of a target object, a very dim region, or a brightness level somewhere in between. Capturing such a large variation of brightness level information can present a challenge to circuit designers. Thus, techniques have been developed to cope with high dynamic range imaging, including the above-mentioned AGC and over-sampling techniques.
But in practice, AGC-based designs are inherently complex and generally significantly increase power dissipation per pixel detector. As a result, AGC-based techniques are difficult to replicate thousands or hundreds of thousands or more times for integration into a large array of pixel detectors. Due to its complexity, AGC is only practical if performed external to the pixel array, which constraint is undesirable because of the inherent speed limitations. Further, AGC requires extra circuitry for a calibration procedure that must be repeated frequently on a per-pixel basis.
As noted, over-sampling techniques are somewhat analogous to over-sampling A/D converters. But a huge stream of pulses is generated when the various pixel detector outputs exceed a threshold, which stream of pulses must be propagated external to the pixel array for collection and further signal processing. Unfortunately, having to propagate the pulse stream external to the pixel array typically creates a processing bottleneck, especially with respect to physically transmitting the content of each pixel externally to the array. While so-called winner-take-all schemes can help, the bottleneck problem remains. Even if propagation is performed on a pixel array column-by-column basis, several nanoseconds may be required for completion, and such process must be repeated for each pixel in the column. System speed performance is constrained not only by information propagation time, but by the number of rows and columns in the detection pixel array, which imposes a limitation on the size of the array.
What is needed is an improved method and system for coping with the high dynamic range encountered during acquisition of information, including two-dimensional data and three-dimensional data. Preferably signal processing techniques including analog/digital processing should be localized within the pixel array, and the conversion results stored for later access and signal processing. The method and system should enable detection of both bright and dim light signals with substantially the same resolution precision without having to readjust or change the mode of operation of the acquisition system. Preferably such method and system should permit re-using circuitry that is already in place in the system. Further, such method and system should be useable, even with very large pixel detection array sizes.
The present invention provides such a method and system.
In a first embodiment, a low noise, readily replicated circuit promotes a large dynamic range of acquired brightness information. Photodetector output I(t) current is input to a variable gain resettable integrator. The integrator output V(t) is input to a comparator for comparison to a threshold voltage Vth. When V(t)xe2x89xa7Vth the comparator changes state. A feedback loop from the comparator output to the bias source for the photodetector helps ensure that the comparator output is a time-lengthened pulse that is input for counting to a reset counter that may be implemented as a sequence chain of latches. Data acquisition ends when the counter attains a desired count and is read, whereupon the entire circuit is reset. Using a high integrator enables the circuit to respond to low amplitude input signals (e.g., dim light signals), whereas resetting the system when V(t) reaches Vth enables the circuit to respond well to large amplitude signals without saturating the integrator.
A second embodiment provides a TOF data acquisition delay locked loop circuit that advantageously can reuse much of the circuitry of the acquired brightness circuit. The TOF acquisition circuit provides a first sequence and a second sequence of series-coupled delay units, a like number of units being in each sequence (or chain). The circuit also includes a like number of latch units. The clock input of a latch unit is coupled to the output of an associated first sequence delay unit, and the data input of the latch unit is coupled to the output of an associated second sequence delay unit. Pulses, which can include photodetector signals that have been integrated and compared against a threshold voltage Vth, can be propagated through one or both chains of delay units. The output from the last delay unit in each chain is input to a phase discriminator, whose output is fed-back to delay units in the second chain and also to the comparator. A control voltage is preferably coupled to each delay unit in the first chain, to vary delay times through the chain.
In this second embodiment, TOF is acquired after a calibration phase during which a GlobalSync pulse train is forced the first and second chain of delay units. During calibration, the phase discriminator forces substantial (but not perfect) equalization of total time delays though both chains. During a measurement phase, a single Sync pulse is propagated to all pixel detectors. When the pulse reaches a pixel detector it is forced through the first chain of delay units, thereby creating a copy of the pulse at various delay times Cx. When an actual light pulse is detected at time tTOF, a photodetector current pulse is generated and integrated and coupled to a comparator until time tTOF+tC OMP. The comparator changes state and the state transition is eventually propagated through the second chain of delay units. The latches coupled between the two chains can capture the precise time at which a light pulse was received from a target object. An embodiment providing successive measurement approximation is used to enhance resolution.